1. Field of the Invention
The present invention relates to a rocket nozzle and a control method for combustion gas flow in a rocket engine.
2. Description of Related Art
FIG. 1 shows a bell-shaped nozzle 100A as a rocket nozzle. When a flow path cross-sectional area at a throat 101A is At and a flow path cross-sectional area at a nozzle exit 102A is A1, an expansion ratio eA of the bell-shaped nozzle 100A is represented by a ratio (A1/At) between A1 and At. FIG. 2 shows a bell-shaped nozzle 100B as a rocket nozzle. When a flow path cross-sectional area at a throat 101B is At and a flow path cross-sectional area at a nozzle exit 102B is A2, an expansion ratio eB of the bell-shaped nozzle 100B is represented by a ratio (A2/At). Here, A2 is greater than A1, and thus eB is greater than eA.
A specific impulse representing a performance of a rocket nozzle is known. The specific impulse varies depending on the expansion ratio and the ambient pressure around the rocket nozzle. Accordingly, the specific impulse changes during the ascent of a rocket.
FIG. 3 is a graph showing changes of specific impulses with respect to altitude. A vertical axis of the graph represents specific impulse and a horizontal axis represents altitude. A performance curve 121 shows the change of a specific impulse of the bell-shaped nozzle 100A with respect to altitude. A performance curve 122 shows the change of a specific impulse of the bell-shaped nozzle 100B with respect to altitude.
The performance curve 121 and the performance curve 122 intersect each other at a certain altitude. The specific impulse of the bell-shaped nozzle 100A is greater than the specific impulse of the bell-shaped nozzle 100B at an altitude lower than the altitude of intersection, and the specific impulse of the bell-shaped nozzle 100B is greater than the specific impulse of the bell-shaped nozzle 100A at an altitude higher than the altitude of intersection.
If an expansion ratio of a rocket nozzle can be changed during the ascent of a rocket, a specific impulse of the rocket nozzle can be kept high over a wide range of altitude.
FIG. 4 shows a dual-bell nozzle 110 as a rocket nozzle. The dual-bell nozzle 110 includes a first stage nozzle 115 as a portion from a throat 111 to an inflection point 112 and a second stage nozzle 116 as a portion from the inflection point 112 to a nozzle exit 113. Each of the first stage nozzle 115 and the second stage nozzle 116 is bell-shaped. Here, a flow path cross-sectional area at the throat 111 is represented by At, a flow path cross-sectional area at the inflection point 112 is represented by A1, and a flow path cross-sectional area at the nozzle exit 113 is represented by A2.
As shown in FIG. 5, when the ambient pressure around the dual-bell nozzle 110 is high, a flow of combustion gas separates from an inner wall surface of the dual-bell nozzle 110 at the inflection point 112. Hereinafter, such flow is referred to as a low expansion flow. An expansion ratio of the dual-bell nozzle 110 under the state of the low expansion flow is approximately equal to the expansion ratio of the bell-shaped nozzle 100A.
As shown in FIG. 6, when the ambient pressure around the dual-bell nozzle 110 is low, the flow of the combustion gas separates from the inner wall surface of the dual-bell nozzle 110 at the nozzle exit 113. Hereinafter, such flow is referred to as a high expansion flow. An expansion ratio of the dual-bell nozzle 110 under the state of the high expansion flow is approximately equal to the expansion ratio of the bell-shaped nozzle 100B.
Referring to FIG. 3, it is optimum that the combustion gas flow in the dual-bell nozzle 110 transits from the low expansion flow state to the high expansion flow state at the altitude corresponding to the intersection of the performance curve 121 and the performance curve 122. The intersection of the performance curve 121 and the performance curve 122 is referred to as an optimum transition point 120. However, as described in “A Critical Assessment of Dual-Bell Nozzles”, G. Hagemann, M. Frey, and D. Manski, 1997, it is known that the combustion gas flow in the dual-bell nozzle 110 transits from the low expansion flow state to the high expansion flow state at an altitude much lower than the altitude corresponding to the optimum transition point 120. For that reason, the change of the specific impulse of the dual-bell nozzle 110 with respect to altitude is represented by a performance curve 123.
Since the state of the combustion gas flow in the dual-bell nozzle 110 transits at the altitude much lower than the optimum transition point 120, the specific impulse of the dual-bell nozzle 110 is reduced at the transition. The lower the altitude at which the transition occurs is, the larger the magnitude of the reduction of the specific impulse becomes. In addition, vibration would be strong because the combustion gas flow tends to separate from an inner wall surface of the second stage nozzle 116 at an upstream position from the nozzle exit 113 when the altitude is low immediately after the transition.